This invention relates to a powdery raw material for use as a starting material for manufacturing anodes of fuel cells, particularly fuel cells of the molten carbonate type, which enables to manufacture fuel cells having such excellent high-temperature creep strength and high sintering resistance as to maintain excellent anode characteristics over a long period of time.
Conventionally, in manufacturing an anode of a molten carbonate type fuel cell (hereinafter merely called "anode"), fine pure nickel powder is employed as the starting material which is generally prepared from nickel carbonyl through decarboxylation thereof and has a mean grain size of 2 microns or less. The fine pure nickel powder is formed into a predetermined shape by a doctor blade method, and then sintered. The sintered body is dipped in an aqueous solution of (NO.sub.3).sub.2 of concentration of 0.1 mol %, then dried and thermally treated to form a chromium oxide layer over the surface of the sintered body to thus obtain an anode. An anode thus manufactured in general has a porous sintered body having a thickness of 1 mm, a porosity of 50-70%, and a mean pore diameter of 7-10 microns.
In the above conventional nickel anode, the chromium oxide layer coated over the sintered body acts to enhance the high-temperature creep strength and sintering resistance of the anode. However, this chromium oxide layer is not sufficient to enhance these properties to a satisfactory degree. As a result, if for example the anode is used under conditions that a load of 2-4 Kg/cm.sup.2 is vertically applied on the anode surface and the temperature of the cell during operation is high, e.g. 650.degree.-700.degree. C. as usually applied, the anode undergoes gradual creep deformation and finally can be broken, which impedes diffusion of a fuel gas into the interior of the anode, resulting in a reduced reaction surface area of the anode and hence increased polarization of the cell, shortening the effective life of the cell.
The terms "creep strength" and "sintering resistance" are both parameters representative of shrinkage of the anode. The term "creep" macroscopically represents the anode shrinkage, i.e. it means an amount of change in the anode thickness, and the term "sintering" microscopically represents the anode shrinkage, i.e. it means a change in the microstructure of the anode. Fuel cells in general undergo gradual sintering and accordingly creep during operation. Since currently there is no suitable means for indicating the degree of sintering in fuel cells, usually the creep deformation is measured and the sintering degree is estimated from the measured creep deformation. Thus, the terms "creep" and "sintering" are synonymous with each other. As sintering grows in a fuel cell, i.e. the anode microstructure changes, it results in degraded cell performance. This degraded cell performance is particularly conspicuous in a stacked fuel cell which undergoes a large change in the total size of the cell due to creep. Therefore, to keep initial cell performance over a long period of time, it is desirable that no sintering and hence no creep should take place in fuel cells.